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Negative Voltage

Discussion in 'Electronic Basics' started by Kev, Aug 8, 2004.

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  1. Kev

    Kev Guest

    Hey Everyone.

    I need to be able to generate a negative voltage from a microcontroller. To
    be more detailed, when the microcontrollers output is 1 (high) I want to
    generate a positive voltage of 9V. But when the output is 0 (low) I want to
    generate a negative voltage of -9V. The positive and negative 9V supply
    must be with respect to the microcontrollers ground. The microcontroller is
    5V and I have a 9V supply (from a 78L09 regulator). I have no negative
    supplies though. The circuit would need to run at speeds of about 70KHz.
    The simpler the solution the better.

    Thanks.
     
  2. Kev

    Kev Guest

    One way I thought I might be able to do it was to use a voltage inverter to
    turn the 9V into -9V and then use some combination of transistors to switch
    between 9V and -9V. Does that make any sense? If it does, could someone
    recommend a circuit?
     
  3. John Fields

    John Fields Guest

     
  4. Byron A Jeff

    Byron A Jeff Guest

    You didn't give the current requirements for the negative supply. Most
    charge pump inverters have a max current of 100mA or less.

    Here's how I'd do it to keep it simple. To generate the negative voltage
    use a standard charge pump, like the cat660, here:

    http://www.catalyst-semiconductor.com/documents/660.pdf

    The part standard is called the 7660. Other parts in this class of chips
    includes the OnSemi 7662 and the Linear Tech LTC1044. Each takes a positive
    voltage (+9V in your case) and inverts it.

    Then you can use an open ended opamp as your switch. Connect the +9V and -9V
    supplies to V+ and V- of the opamp. In its most basic form an opamp will drive
    the output so that the two input voltages match. Typically feedback is used
    to accomplish this where the output is fed back into one of the inputs.
    However in an open ended setup, the output will try to drive the voltage
    towards the rails. So the basic rule is that the output will drive positive
    as long as the + input is greater than the - input, and vice versa. So
    here's how you can set up it. Use a voltage divider to get a 2.5V to the
    - input of the opamp. This can be as simple as two equal 100k resistors
    connected between +5 and GND. The connect the microcontroller output to the
    + input of the omp amp. When the uC output is +5 the + input is above the
    - input so the ompamp output will swing to +9V. OTOH when the microcontroller
    output is 0V the + input is below the - input and the opamp output will drive
    to -9V. Problem solved.

    Be sure to get a rail to rail opamp for the job.

    So two ICs, two resistors, and two caps. Not two ;-) much.

    BAJ
     
  5. Rich Webb

    Rich Webb Guest

    What's your tolerance on that 9 V? Something like a MAX233 will give you
    a swing of about +/- 8 V with just one chip and no external components.
    It also inverts, of course.
     
  6. That is what I was thinking of, but I have no idea if you need any
    regulation of the +-9 or if crude copies of the 5 volt regulator input
    voltage is okay. What current will load these 9 volt outputs?
     
  7. Jamie

    Jamie Guest

    assuming that you don't have a +/- supply i guess you could
    construct a mini op AMP osc with POS feed back then Rectify that via
    CAP and Diode.
    the output can be directly..
    you could use an isolation diode to switch the osc on when the
    out goes low.
    etc..
     
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